This invention relates generally to digital signal receivers, and more particularly to adaptively equalizing received digital signals.
Recently, television broadcasters have begun transmitting digital television (DTV) signals as a packetized data stream which is standardized by the Advisory Committee on Advanced Television Service, see xe2x80x9cATSC Digital Television Standard,xe2x80x9d 1995. In the United States, trellis coded vestigial sideband modulation (8 VSB) is typically used for DTV transmissions, see xe2x80x9cVSB Transmission Systemxe2x80x9d, Zenith Corporation, pp. 1-25, 1995. For cable systems, digital television broadcasting will use QAM or 16 VSB modulation.
DTV transmission over terrestrial broadcast channels poses special problems. The high data rates, coupled with time-varying channel responses spanning hundreds of symbols, implies that high performance demodulation is difficult to achieve. A particular challenge for terrestrial DTV television receiver designers is correcting static and dynamic multipath impairments. A typical receiver is expected to handle multipath with delays as long as 18 microseconds.
A decision feedback equalizer (DFE), because of its relatively low complexity and near optimal performance, is potentially a good solution to the problem. However, even the DFE can be overly complex when delay spreads extending over hundreds of symbols are encountered. After all, a DFE can only cancel multipaths up to the number of taps in the feedback filter.
At a symbol rate of 10.76 Mbits per second, adaptive equalizers with lengths of several hundred taps are required. In addition, to combat dynamic multipaths due to propagation effects such as flutter from moving objects, e.g., airplanes, and changing atmospheric conditions, the equalizer must update its coefficients at a high speed.
To meet this challenge, prior art equalizers are typically implemented in hardware. This means that a large number of arithmetic units are required, Thus, prior art equalizers are relatively highly complex. Complexity implies processing delays. In addition, the power consumption of such equalizers is very high, due to the large number of concurrently operating arithmetic logic units.
Therefore, it is desired to provide a method an apparatus for adaptively filtering digital signals, with shorter acquisition times to better respond to dynamic multipath impairments.
Provided are a decision feedback equalizer (DFE) method and apparatus for digital receivers. In one aspect, the equalizer according to the invention exploits the sparse channel characteristics of terrestrial digital television transmission, and the profile of multipath impairments. The invention uses a least mean square (LMS) method for updating equalizer coefficients.
The complexity of coefficient updating is reduced as follows. The step size of a coefficient updating process is multiplied by a weighting parameter w. The value of w can be either 0 or 1, depending on the equalizer tap location. Only taps with substantial energy and their neighboring taps have w equal to 1, the remaining taps have w equal 0. Those taps with zero weighting are not updated. The locations of non-zero weighting taps are determined during initialization, when all taps are updated for several iterations so that tap energy level can be calculated. The energy of the non-zero weighting taps is continuously monitored. Zero weighted taps may need to be updated later if dynamic ghosts are detected.
In another aspect of the invention, dynamic ghosts are tracked. For taps with w equals to 0, the invention applies a threshold update step to update w for those taps that are affected by dynamic ghosts. A convergence quality Q, defined as the difference in RMS values between current iteration and the running average RMS values for the past N iterations, is used to gauge the direction of the convergence, and to update the step size of the coefficient updating.
More specifically, in a digital receiver, an adaptive decision feedback equalizer receives signals having multipath interference. The equalizer includes a feed-forward, a feedback, and a partial feedback filter. The equalizer also includes a slicer and two accumulators. An output from the slicer is processed by the feedback filter and the partial feedback filter. The output from the feedback filter is subtracted from output of the feed-forward filter to form the input to the slicer, and the output of the partial feedback filter is subtracted from the received signal to form the input for the feed-forward filter to reduce the multipath interference.